Hey Guys,

can anyone please give me some advice?im looking at buying my ford 9" diff for my ute build(ca18det with 5 speed rb25det gearbox)and was wondering what ratio I should go for any advice would be great.Would 4.11 be the go?

Cheers.

4.11s are definitely a common choice -- high enough for spritely acceleration but not so high as to be noiser/fuel guzzler/limited top speed/ than most cars. My Subaru Forester has factory 4.11 gearing (non-turbo engine). It gets worse fuel economy than most other similar year vehicles but has better performance because of it

If you want maximum acceleration, the lower the gear the better. 4.625 would be better than 4.11. If you want maximum acceleration in the 1/4 mile drags, there is a limit to this. You want to optimize RPM at the finish line, say 6000 RPM in 4th gear. This depends on the engine tune (kW and Torque) and the tire size you will be running. If you go too low (say 4.825) you may run out of RPM in fourth gear. Or maybe you'll get the fastest drag times with 4.88 gears and taller slicks

Or do you want relaxed highway driving? Higher gears are better say 3.7 is quieter and more leisurely than 4.11 and allows you to "stay in gear" longer before having to shift. The more engine torque you have the higher gear you can go without losing to much performance. 3.454 is available for the H165 and allows for best fuel economy and quietest (lowest RPMs) highway cruising. A 68hp A12 would be slow accelerating with 3.454 gears but a larger engine can do it with acceleration times that are no-slower-than-factory

ca18det_man was using 4.625 with his CA18DET
Quote:

Hey DD,


I don't really plan on drag racing or circuit racing the beast,just would like a niece street car but will be running 3-400hp.

Do the math; use one of those online gearing calculators and figure out what the RPM will be at highway speed in 5th gear.

Normally I'd say 4.11 but if you're pumping out 300hp it'd likely pull a 3.9 easily. The lower revs at highway might be worthwhile.

Hey Rallytwit,

either 3.9 or 4.11 I think would be the go,gives me around the 3500rpm at 120kph.